1. Field of the Invention
The present invention relates to a semiconductor mask, and more particularly, to a phase shift mask and a method for fabricating the phase shift mask.
2. Background of the Related Art
A photomask having a part for transmitting light and another other part for shielding light is generally used in photolithography, which is used for fabricating a semiconductor device. However, the related art photomask, which has a light transmission pattern and a light shielding pattern for selective exposure, has a limited resolution because of diffraction. To decrease the diffraction, the phase shift mask combines a light transmissive region, which transmits it, and a light shifting transmissive region, which transmits a light shifted by 180 degrees.
FIGS. 1a-1d illustrate sections showing one example of the steps of a related art method for fabricating a phase shift mask. Referring to FIG. 1a, a light shielding layer 2 and a photoresist 3 are successively formed on a light transmissive substrate 1. The photoresist 3 is subjected to exposure and development to define regions in which first and second light transmissive regions are to be formed and to pattern the photoresist 3 to form first and second photoresist holes H1 and H2 therein. The first photoresist holes H1 are used to form the first light transmissive regions and the second photoresist holes H2 are used to form the second light transmissive regions.
Referring to FIG. 1b, the patterned photoresist 3 is used as a mask in etching the light shielding layer 2 and the light transmissive substrate 1 to a predetermined depth to form a first light transmissive region 4 and second light transmissive regions 5. Then, the photoresist 3 is removed. The etching depth of the substrate 1 is predetermined so that a phase of light passing therethrough is shifted by 160-200 degrees compared to a phase of light which passes through the light transmissive substrate 1. The first light transmissive region 4 is a light transmissive region that actually patterns a photoresist, and the second light transmissive regions 5 are light transmissive regions that do not actually pattern the photoresist. The light passing through the two kinds of regions are shifted by 160-200 degrees each in phase.
Referring to FIG. 1c, on an entire resultant surface, a photoresist 6 is deposited and patterned to selectively remove the photoresist 6 on both sides of the first light transmissive region 4. At the same time, the photoresist 6 between the second light transmissive regions 5 is also removed. Each of the removed widths of the photoresist 6 on both sides of the first light transmissive region 4 and the second light transmissive region 5 is the same.
Referring to FIG. 1d, the photoresist 6 is used as a mask in a selective etching of the light shielding layer 2 to form a third light transmissive region 6 and fourth light transmissive regions 7. The third light transmissive region 6 is a light transmissive region that actually carries out patterning of a photoresist, and the fourth light transmissive regions 7 are light transmissive regions that do not actually carry out patterning of the photoresist.
The related art phase shift mask includes a first light transmissive pattern 8 which includes a first light transmissive region 4, which is a main light transmissive region, and the fourth light transmissive regions 7, which are supplementary light transmissive regions. The shifting of a phase of light passing through the fourth light transmissive regions 7 of the supplementary regions opposite to a phase of light passing through the first light transmissive region 4 of the main light transmissive region prevents an intensity of light passing through the main light transmissive region from becoming gentle by a side lobe at a boundary, resulting in a steep slope of the light intensity, thereby allowing formation of an accurate desired pattern.
A second light transmissive pattern 9 adjacent to the first light transmissive pattern 8 includes the third light transmissive region 6, which is a main light transmissive region, and second light transmissive regions 5, which are supplementary light transmissive regions. The operation of the second light transmissive pattern is identical to the first light transmissive pattern. However, as the major phases of the second light transmissive pattern 9 and the first light transmissive pattern 8 are opposite to the other, their side lobes at an overlapped portion d1 are canceled by the other, thereby preventing the formation of an anomalous pattern.
FIGS. 2a-2f illustrate sections showing a second example of the steps of a related art phase shift mask. Referring to FIG. 2a, a light shielding layer 11 and a first photoresist 12 are successively formed on a light transmissive substrate 10 having light transmissive pattern regions P.sub.1 and P.sub.2 and a light shielding pattern region P.sub.3 defined thereon. The P.sub.1 region is a region in which a first light transmissive pattern is to be formed, the P.sub.2 region is a region in which a second light transmissive pattern is to be formed, and the P.sub.3 region is a region in which a light shielding pattern is to be formed.
Referring to FIG. 2b, the first photoresist 12 at the edges or rims of the first and second light transmissive pattern regions P.sub.1 and P.sub.2 are selectively patterned. The patterned first photoresist 12 is used as a mask to etch the light shielding layer 11 and the light transmissive substrate 10 to a predetermined depth for forming a plurality of light transmissive holes H.sub.10.
Referring to FIG. 2c, the first photoresist 12 is removed. Then, on an entire resultant surface, a second photoresist 13 is deposited. The second photoresist 13 in the first and second light transmissive regions P.sub.1 and P.sub.2 is selectively patterned with exposure and development. The second photoresist 13 fully covers the light shielding layer 11 in the light shielding pattern region P.sub.3, only to expose the light shielding layer 11 in the light transmissive pattern regions P.sub.1 and P.sub.2.
Referring to FIG. 2d , the second photoresist 13 is used as a mask to etch the exposed light shielding layer 11. Then, the second photoresist is also removed. By this, the first light transmissive regions 14, which are the main light transmissive regions, and second light transmissive regions 15, which are formed at the rims of the first light transmissive regions 14, are formed repeatedly on the light transmissive substrate 10 having the first and second light transmissive regions P.sub.1 and P.sub.2 defined thereon.
Referring to FIG. 2e, a third photoresist 16 is deposited on an entire resultant surface, and the third photoresist 16 in only one of the first and second light transmissive pattern regions P.sub.1 and P.sub.2, for example, the second light transmissive pattern region P.sub.2 is selectively patterned.
Referring to FIG. 2f, the third photoresist 16 is used as a mask in simultaneously etching the first light transmissive region 14 and the second light transmissive regions 15 in the second light transmissive pattern region P.sub.2 to predetermined depths to form a third light transmissive region 17, which is a main light transmissive region, and fourth light transmissive regions 18, which are supplementary regions.
The etching depths of the third light transmissive region 17 and the fourth light transmissive regions 18 are determined so as to shift phases of the light passed through the third light transmissive region 17 and the fourth light transmissive regions 18 by 160-200 degrees and 360 degrees, respectively, compared to the phase shiftings of light passing through of the first light transmissive region 14 and the second light transmissive regions 15 which are used as reference for 0 degrees and 180 degrees, respectively. Since the phases of lights of the first light transmissive pattern P.sub.1 and the second light transmissive pattern P.sub.2 are opposite to the other, their side lobes which can be formed at a location of the light shielding layer 11 pattern in an overlapped portion of the two light transmissive patterns P.sub.1 and P.sub.2 are canceled by the other, thereby preventing the formation of an anomalous pattern.
FIG. 3 illustrates a section showing a third example of the phase shift mask. A light shielding layer 22 has a plurality of first light transmissive regions 21 formed on a light transmissive substrate 20. The light shielding layers 24 are formed on the light shielding layer 22 on both sides of the second light transmissive region 23. The first light transmissive region 21, the second light transmissive region 23 and the light shielding layers 24 are repeatedly formed on the substrate 20. When a phase of the light passing through the first light transmissive region 21 has a 0 degree phase shift, the second light transmissive region 23 transmits the light of 180 degrees phase shift. Therefore, by using a phase shift mask having the first light transmissive region 21 and the second light transmissive region 23 alternately formed, formation of side lobes caused by diffraction of lights can be prevented.
The above phase shift masks and methods of making the masks have various disadvantages. The etching processes required for etching the light transmissive substrates in formation of the main light transmissive patterns and the supplementary patterns may damage the substrates, and it may be difficult to control their etching stop points. Further, the etching of a phase adjusting member of the light transmissive substrate to the maximum etching depth, i.e., depth to provide a 360 degree phase shift, causes a change in its light intensity, which degrades the reliability. The process in which a surface of the light transmissive substrate is exposed partially damages the substrate, which may cause phase errors.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.